The present invention relates to a mobile communication system using a code divisional multiple access (CDMA) site diversity system in which a mobile station makes simultaneous communication through two or more base stations.
A conventional mobile communication system of the type described comprises a mobile station, a number of base stations, and a mobile exchange. The mobile station transmits packet data during communication. The packet data are received by the individual base stations. Each base station receives the packet data and sends it to the mobile exchange as received packet data.
The mobile exchange comprises a data quality decision unit, a data selecting unit, and a data processing unit. The data quality decision unit and the data selecting unit receive a number of received packet data simultaneously from a number of base stations. The data quality decision unit compares these received packet data for quality in response to the reception of them from the base stations. The data quality decision unit then supplies a comparison result to the data selecting unit and the data processing unit. The data selecting unit selects one received packet data having the best quality of these received packet data according to the comparison result. The data selecting unit then supplies it as selected packet data to the data processing unit. The data processing unit carries out a predetermined code conversion operation to convert the selected packet data and produces it as converted packet data.
A number of the received packet data received simultaneously by the two or more base stations may all be bad in quality in such mobile communication system. This means that all the received packet data have a frame error or errors. The data quality decision unit thus supplies an error detection signal to the data selecting unit and the data processing unit, indicating that all the received packet data have the frame error. In response to the error detection signal, the data selecting unit rejects or turns down all the received packet data transmitted from the base stations. No selected packet data is thus produced from the data selecting unit. In this case, the data processing unit carries out processing to complement the selected packet data in response to the error detection signal. This processing is achieved by means of carrying out a predetermined code conversion operation using the selected packet data previously received and producing it as the converted packet data. Alternatively, this processing may be achieved by means of carrying out a predetermined code conversion operation using packet data to be recognized as silent or background noise data and producing it as the converted packet data. In such a case, the communication may be broken instantaneously or be suffered from a short-time noise, with a detrimental effect on the quality of the communication.